New device determines fluid concentrations

By James E. Kloeppel

UI researchers have invented a simple, low-cost device that monitors
the concentrations of fluid mixtures by detecting small changes in the way
the mixtures bend light rays. The instrument ­ a new type of refractometer
­ can be used to automatically measure the amount of oil in automobile
refrigerants, the concentrations of aqueous coolant brines used in industry
and a variety of process fluids.

"The device works by measuring the amount of bending that occurs
when light enters a fluid," said Ty Newell, a professor of mechanical
engineering and a researcher affiliated with the university's air conditioning
and refrigeration center. "The extent to which light rays are bent
is determined by a property of the fluid called the refractive index."

To measure the refractive index, Newell and graduate student Evan Hurlburt
mounted a light-emitting diode to one side of a semi-transparent piece of
glass. The other side of the glass is placed in contact with the fluid to
be measured.

"Some of the light rays are transmitted through the fluid,"
Newell said. "Other rays, depending upon the refractive index of the
fluid, are bent back toward the glass. This results in a sharp ring of light
being formed at a specific distance from the light source. By measuring
the diameter of the ring, we can determine the refractive index of the fluid."

If the fluid concentration changes, the refractive index also will change,
causing the ring of light to shift in size and intensity. A cadmium-sulfide
photoresistor, cemented to the glass surface with a clear epoxy, can monitor
the ring's appearance and provide a continuous measurement of the refractive
index. (A temperature sensor, also cemented to the glass, takes into account
any temperature variations that can affect the refractive index.)

Both portable and in-line versions of the device have been developed,
Newell said. "For example, we constructed the coolant-brine refractometer
as a hand-held probe that can be inserted into the coolant, but we built
the automotive refractometer into a compact pressure housing designed to
withstand harsh operating conditions."

Installed in an automotive air-conditioning system, the refractometer
could sense a drop in the amount of oil that circulates with the refrigerant
and automatically shut down the system to prevent compressor failure, Newell
said.

Because the refractometer can measure subtle changes in fluid concentrations,
the device could be used in a number of other applications as well, including
monitoring additives in food and beverage processing, measuring the charge
content of lead-acid storage batteries and detecting leaks from underground
storage tanks.